REVIEWS
MODIFIER GENES IN
MICE AND HUMANS
Joseph H. Nadeau
An emerging theme of studies with spontaneous, engineered and induced mutant mice is
that phenotypes often depend on genetic background, implying that genetic modifiers have
a role in guiding the functional consequences of genetic variation. Understanding the
molecular and cellular basis by which modifier genes exert their influence will provide insights
into developmental and physiological pathways that are critical to fundamental biological
processes, as well as into novel targets for therapeutic interventions in human diseases.
INBRED STRAINS
Mouse strains generated
through systematic inbreeding,
which results in virtual genetic
homozygosity.
Department of Genetics,
Case Western Reserve
University School of
Medicine and Center for
Human Genetics,
University Hospital of
Cleveland, 10,900 Euclid
Avenue, Cleveland, Ohio
44106, USA.
e-mail: jhn4@po.cwru.edu
It is becoming increasingly evident that traits that are
inherited in a simple Mendelian fashion can have phenotypes that differ in subtle or profound ways1–6. Many
Mendelian traits that result in birth defects, adult diseases and other genetic disorders vary in diverse biological features such as age-of-onset, severity and other
associated phenotypic properties. In fact, the consistent
expression of traits seems to be the exception in humans
and experimental species such as laboratory mice.
Among the several causes of variable phenotypes for
Mendelian traits are alternative alleles, environmental
factors and modifier genes. Examples of allelic and environmental variability are numerous and well characterized4,7–11. Evidence for modifier effects comes from a
range of phenotypes in human families or among different INBRED STRAIN backgrounds that cannot be
explained by alleles of the disease gene or by environmental factors, and when the presence of at least one
independent gene accounts for the modified phenotype.
Although examples of genetic modifiers are limited in
humans, they are frequent in laboratory mice and are
expected to be a common cause of phenotypic variability in humans.
Phenotype modification occurs when the expression
of one gene alters the expression of another gene1,2.
Modification could involve any aspect of a trait, from
the primary action of the target gene (transcription),
through to intermediate phenotypes at the molecular or
cellular levels, to end phenotypes at the organ, system or
organismal levels. Targets of modifier effects could be
spontaneous mutations, engineered mutations produced by transgenesis or gene targeting, or chemical- or
radiation-induced mutations. In laboratory animals,
modifier effects are usually attributed to genetic background and can be inherited as Mendelian or polygenic
traits. In most cases, the genetic basis for modification is
unknown; in some cases, modifier genes have been
mapped; in several cases, candidate genes for modifier
effects are being evaluated; and in a few cases modifier
genes have been identified.
These modifiers provide insight into genes that
underlie diverse developmental and physiological
processes. Direct interactions between proteins can be
discovered by using the yeast two-hybrid system12, mass
spectroscopy13 and other related methods. But genetic
studies remain one of the most powerful ways to find
both indirect and direct interactions. In this review, I
discuss the various kinds of modifier gene and their
phenotypic effects in mice and humans, and evaluate
how modifier studies in model organisms can inform
research into modifiers that determine the severity of
human diseases.
Modifier effects
Genetic modifiers can affect penetrance, dominance
modification, expressivity and pleiotropy (FIG. 1).
Depending on the nature of the phenotypic effect, modifiers might cause more extreme (enhanced) phenotypes,
less extreme (reduced) phenotypes, novel (synthetic)
phenotypes or wild-type (normal) phenotypes, as dis-
NATURE REVIEWS | GENETICS
VOLUME 2 | MARCH 2001 | 1 6 5
© 2001 Macmillan Magazines Ltd
REVIEWS
Genotype
Strain A
Strain B
Strain C
+/+
m/+
m/m
a Reduced
penetrance
c Expressivity
c Expressivity
d Pleiotropy
b Dominance
modification
c Expressivity
Figure 1 | Types of modification.The m mutation arose on the Strain A background in which
it has recessive effects on variegation of the coat colour. The mutation was then transferred
to the Strain B and C backgrounds by repeated backcrosses and by either genotypic or
phenotypic selection. The resulting congenic strains have novel phenotypes that involve
penetrance, dominance modification, expressivity and pleiotropy. a | Penetrance measures
the frequency of affected mice among carriers. In mutant homozygotes, penetrance is 100%
on the Strain A and Strain C backgrounds, but is reduced to 75% on the Strain B background,
regardless of the nature of the variegated coat. b | Dominance modification describes the
change in dominance. On the Strain A and Strain B backgrounds, none of the m/+
heterozygotes is phenotypically affected, whereas on the Strain C background, all
heterozygotes are affected, indicating a change from a recessive trait to a semidominant trait.
c | Expressivity describes the extent to which a trait is affected in mice with a variegated
coat colour. Genetic modifiers from the Strain B and C backgrounds change the pattern
of variegation from coarse, as on the Strain A background, to fine. d | Pleiotropy describes
the variety of traits in affected mice with the mutant gene. On the Strain A and Strain B
backgrounds, mutants show only the variegated coat, whereas on the Strain C background,
mutants show a change in body shape, as well as a variegated coat.
CONGENIC STRAINS
Mouse strains in which a
chromosome segment from
one inbred strain background
has been transferred to
another inbred strain
background by repeated
backcrossing and selection,
either for the phenotype of
interest or for genotypes of
marker loci that flank the
mutant gene of interest.
166
cussed below and in FIG. 1. Epistasis, which occurs when
an allele of one gene masks the phenotype of another
gene14,15, is also a form of genetic modification. To distinguish genetic modifier effects from other forms of phenotypic modification, we assume that individuals of the
same genotype are evaluated on different genetic backgrounds but under similar environmental conditions.
Penetrance is a measure of the frequency of affected
individuals among the carriers of a particular genotype.
It is therefore a characteristic of a population of individuals and can often be reduced among individuals with
the same genotype for a particular Mendelian trait. In
mice, the presence of modifier genes that affect pene-
trance becomes evident when the frequency of affected
mice varies among CONGENIC STRAINS in which the same
mutant allele has been transferred to different genetic
backgrounds, or among segregating crosses that involve
different combinations of strains (FIG. 1a). In humans,
individuals with a ‘disease’ genotype who are nevertheless unaffected are evidence of reduced penetrance. If
linkage analysis of these unaffected individuals reveals
evidence for an independent gene, the basis for reduced
penetrance is a genetic modifier.
Dominance depends on many factors, including
alleles, genotypes, genetic background and environmental conditions. Dominance of a particular genotype
often varies with genetic background, even under similar environmental conditions. A trait that is inherited in
a dominant manner on one background can be inherited as a semidominant or recessive trait on another background (FIG. 1b). The ability of genetic background to
modulate dominance, under the same environmental
conditions, suggests that independent genetic factors
determine whether mutant heterozygotes are affected.
Expressivity describes the extent to which particular
structures or processes are affected by a particular genotype. The trait might be enhanced or reduced; it might
even be suppressed so that mutants seem to be almost
normal. The action of genetic modifiers is evident when
the extent to which a trait is affected varies among
genetic backgrounds (FIG. 1c). Expressivity is relevant
only in individuals in which the trait is penetrant.
Pleiotropy refers to the diverse phenotypic effects of
a single-gene mutation. Genetic modifiers of pleiotropy
result in different combinations of traits on different
genetic backgrounds (FIG. 1d). These modifiers can also
lead to novel phenotypes that are found only on particular genetic backgrounds. Such a mutation might affect
structures and functions that seem to be developmentally and physiologically unrelated, the interrelationships of which might only become evident once the
gene defect is identified.
Environmental factors that affect the expression of a
trait can be mistaken as an effect of genetic modifiers.
These factors can cause a trait to be strongly expressed
in one environment but not to be expressed in another
environment. Distinguishing genetic and environmental modifiers, although complicated, can be resolved if
genetically identical individuals are evaluated on different genetic backgrounds in similar environments. It is
therefore essential that modifier genes are studied under
similar environmental conditions, or at least that nongenetic effects are taken into account.
FIGURE 2 shows the ways in which background genes
modify pleiotropy, penetrance, dominance modification and expressivity to modulate phenotypes. The key
components of this model are: first, the distribution of
trait values for individuals of the three genotypes at the
modified gene; and second, the threshold that determines whether the trait is expressed in individuals of a
particular genotype. Genetic variation in the modifier
gene affects the position of the phenotypic threshold
relative to the trait distributions. By contrast, genetic
variation in the modified target gene affects the position
| MARCH 2001 | VOLUME 2
www.nature.com/reviews/genetics
© 2001 Macmillan Magazines Ltd
REVIEWS
Threshold for
trait expression
Number of individuals
a
+/+
+/– –/–
+/–
+/+
–/–
–/–
+/+
Trait distribution
New
Old
Trait 2
Threshold for
trait expression
b
Number of individuals
d Trait 1
+/–
+/–
+/–
+/+
–/–
–/–
+/+
Trait 3
Trait distribution
New
+/+
+/– –/–
Old
+/–
Number of individuals
c
–/–
+/+
–/–
–/–
Trait distribution
Threshold
for trait
expression
e
More extreme
Number of individuals
Trait distribution
Figure 2 | A model of modification. This model illustrates
+/–
how alleles of modifier and modified genes affect penetrance,
Old
New
dominance modification, expressivity and pleiotropy, and
mean
mean
modulate the phenotypes of mutant organisms. Throughout
+/+
–/–
–/–
this figure, the solid curves and lines show the distribution of
trait values without the modifier, the dashed curves and lines
show the values with the modifier and the vertical line shows
the phenotypic threshold. a | Penetrance. Modifiers move the
Trait distribution
Phenotype
threshold for expressing the trait relative to the distribution of
threshold
trait values. By moving the threshold, a greater (or smaller)
proportion of the mutant homozygotes are affected, thereby increasing (or reducing) penetrance.
b | Dominance modification. Modifiers move the threshold for expressing the trait relative to the distribution of trait values. By moving
the threshold into (or out of) the range of the heterozygotes, some mutant heterozygotes are (or are no longer) affected, thereby
converting a trait with recessive effects into one with semi-dominant effects (or the converse depending on the direction in which the
threshold moves). c | Expressivity. The location of the distribution of trait values for mutant homozygotes or mutant heterozygotes,
or both, but not for the wild-type homozygotes is shifted relative to the phenotypic threshold. d | Pleiotropy. Modifiers determine the
particular traits that are affected in mutant organisms. Any of three traits can be found in individuals with a mutant gene, depending
on the action of a modifier gene. When the modifier is absent, only traits 2 and 3 are observed, whereas when the modifier is
present, only traits 1 and 2 are observed. Only expression of trait 2 is independent of the modifier. e | Phenotypic changes resulting
from different alleles of the disease gene. In this case, modifier genes are not involved. The alternative alleles result in a slight shift in
the distribution of trait values for the heterozygotes and a more dramatic shift for homozygotes.
of the trait distribution of the mutant homozygotes,
heterozygotes or both relative to the phenotype threshold (FIG. 2e), which is assumed to be fixed in cases for
which only genetic variation in the modified gene is
considered. Additive effects can be distinguished from
modifier effects by whether the distribution of trait values is affected for all genotypes or only for selected
genotypes, because only additive effects shift the distribution of trait values for all genotypes. Although it does
not explain the mechanisms of modification, this model
outlines the various ways in which modifiers modulate
fundamental genetic properties to influence the expression of phenotypic traits.
Modifier genes in mice
Many examples of modifier effects can be found in the
classical literature of mouse genetics1,2. Recently,
mutants generated by gene targeting have revealed
NATURE REVIEWS | GENETICS
VOLUME 2 | MARCH 2001 | 1 6 7
© 2001 Macmillan Magazines Ltd
REVIEWS
Table 1 | Examples of modifier genes and their phenotypic effects
Target modified gene
Modifier gene
Modifier effect
Nature of the
modified phenotype
References
disorganization (Ds)
Genetic background
(C57BL/6J or C3H)
Penetrance
Whether or not an affected
mouse shows Ds-like
birth defects
16
undulated (Pax1un)
Genetic background
(C57BL/6J × CBA)
Dominance modification
Presence or absence of
foramina transversaria
19, 20
short-ear (Bmp5se)
Genetic background
(C57BL/6J or CBA)
Dominance modification
Presence or absence of
foramina transversaria
19, 20
brachyury (T)
Genetic background
(selection experiment
based on mixed
background)
Expressivity
Tail length
23
Cftr knockout
Cfm1
Suppressor
Suppresses meconium ileus
65
Apcmin
Phospholipase A2,
group IIA (Mom1)
Suppressor
Reduces polyp number
70
Apcd716
Cox2 (knockout)
Suppressor
Reduces polyp number
78
Apcd716
cPLA2
Suppressor
Reduces polyp number
79
Pax3Sp
Fidgetin — fidget (Fignfi)
Suppressor
Suppresses spina bifida
26
ashen (Rab27a)
dsu
Suppressor
Coat colour suppressor
29
leaden (ln)
dsu
Suppressor
Coat colour suppressor
29
ruby-eye (ru)
dsu
Suppressor
Coat colour suppressor
29
ruby-eye 2 (ru2)
dsu
Suppressor
Coat colour suppressor
29
Patch (Ph)
Pax1un
Novel phenotype
Spina bifida occulta
in Ph/+ un/un mice
32
Pax3Sp
Curly tail (ct)
Novel phenotype
More extreme spina bifida
In Sp/+ ct/ct mice
44
Examples in mice (allele)
Examples in humans (gene/disease)
Peripherin 1 (PRP1)
ROM1
Dominance modification
Retinitis pigmentosa
in PRP1/+ heterozygotes
47
Familial hypercholesterolaemia
(FH)
Unnamed modifier
on 13q
Suppressor
Reduces cholesterol level
in FH
50
Familial Mediterranean
fever (FMF)
Serum amyloid A
(SAA)
Pleiotropy
Whether FMF cases
show renal amyloidosis
51
Cystic fibrosis transmembrane
conductance regulator (CFTR)
Cystic fibrosis (CFM1)
Suppressor
Suppresses meconium
ileus
66
Non-syndromic deafness
(DFNB26)
At least one gene
on chromosome 7
Penetrance
Suppresses deafness
45
Mitochondrial 12S ribosomal
gene
At least one gene
near D8S277
Penetrance
Suppresses deafness
46
(Apc, adenomatous polyposis coli; Cfm1, cystic fibrosis modifier 1; Cox, cyclooxygenase; cPLA2, cytosolic phospholipase A2; dsu, dilute suppressor; Pax, Paired box
transcription factor; ROM, rod outer segment protein.)
many important examples of modifier genes. These and
other examples show that modifier effects are common
and involve most aspects of mouse biology (TABLE 1).
SYNDACTYLY
Fused digits.
POLYDACTYLY
Supernumerary digits.
FORAMINA TRANSVERSARIA
IMPERFECTA
An opening for nerves and
blood vessels in the transverse
process of the cervical
vertebrae. The presence and size
of this aperture (called a
foramen) is variable.
168
Penetrance. The phenotype of mice with the disorganization (Ds) mutation is an example of a trait in which
modifiers affect penetrance but not other aspects of the
phenotype. Penetrance varies considerably among
inbred strains, from almost zero on the C57BL/6J
inbred background to 10% on the A background to as
high as 89% on the C3H background16. Disorganization
also has perhaps the most striking variation in expressivity of any gene in the mouse16,17. Each affected mouse
has unique birth defects, and no two affected mice have
identical phenotypes. Any body part can be affected in
diverse manners from agenesis to malformation and
extranumerary structures. Features such as extra, missing, cystic or fused kidneys, SYNDACTYLY or POLYDACTYLY, or
missing or ectopic limbs are common. Variable expression, which is an inherent feature of the mutation, is
seemingly unaffected by genetic background and apparently results from random somatic events18.
Dominance modification. A classic example of dominance modification involves a variable feature of the
cervical vertebrae. These vertebrae have FORAMINA TRANSVERSARIA IMPERFECTA (FTI), with variability in the presence
and size of the foramen, depending on the Mendelian
mutant that is present and on the genetic background.
In both undulated and short-ear mice, which are caused
| MARCH 2001 | VOLUME 2
www.nature.com/reviews/genetics
© 2001 Macmillan Magazines Ltd
REVIEWS
by mutations in Pax1 and Bmp5, respectively, the presence of FTI in heterozygotes depends on genetic background. For example, FTI are common in short-ear heterozygotes on the C57BL/6J background, but are absent
in short-ear heterozygotes on the CBA background19,20.
A recent example of dominance modification involves
the modifier of deaf waddler (mdfw) gene, which modifies hearing loss in deaf waddler heterozygotes21.
SPINA BIFIDA OCCULTA
Failure of vertebral fusion but
when skin covers the open
arches and without the
protrusion of a hernia.
KYPHOSIS
The abnormal curvature of the
thoracic spine.
RACHISCHISIS
A form of spina bifida in which
the spinal cord is split.
ANENCEPHALY
Absence of the greater part of
the brain, often with skull
deformity.
Expressivity. Mice that are heterozygous for the T mutation in the brachyury gene have a short tail and homozygotes die during embryonic development22. Although
T/+ heterozygotes usually have a short tail, the extent of
tail-shortening varies considerably among genetic backgrounds. On a mixed background, the tail length in
some T/+ heterozygotes is very similar to that of their
wild-type siblings, whereas other T/+ heterozygotes have
an exceptionally short tail. Selection experiments starting
with a genetically mixed founder population show that it
is possible to select a line in which most T/+ mice have a
very short tail and another line in which most T/+ mice
have a nearly normal tail23. These modifiers did not affect
the length of the tail in the wild-type controls, showing
that the experiment did not simply select for longer tails
in both wild-type and T/+ mice. So, genes in different
strains can have profound consequences on the expressivity of the short-tail phenotype.
An example of a modifier gene that normalizes a phenotype can be seen in the interaction between the splotch
and fidget mutations. Splotch results from a mutation in
the paired box gene Pax3, and homozygous Pax3Sp
embryos die mid-gestation with neural tube defects.
Fidget results from a mutation in the fidgetin (Fign)
gene24. Fidget mutant homozygotes show head-tossing,
circling, hypersensitivity to sound, corneal lesions,
absence of the bony labyrinth and semicircular canals of
the inner ear, and other neurological and morphological
abnormalities25. In splotch–fidget double-mutant
homozygotes, both the incidence and severity of neural
tube defects are greatly reduced26. The contrasting effects
of the splotch and fidget mutations on cell-cycle dynamics might restore normal development to the neural
tube26. The splotch mutation is thought to increase the
length of the cell cycle27, whereas the fidget mutation is
thought to decrease it26. The combination of the two
mutations in double homozygous mutant mice could
restore normal cell-cycle dynamics. Now that both the
spotch and fidget genes have been cloned, it should be
possible to revisit this unique modifier effect and determine the molecular and cellular basis for the interaction.
Another example of a modifier gene that normalizes
phenotypes involves the dilute suppressor mutation.
Dilute is one of the classic coat-colour mutations in the
mouse. It is a recessive mutation that causes a lighter
coat colour in mice with a pigmented coat and results
from mutations in the myosin 5a (Myo5a) gene. In mice
that are homozygous for the dilute suppressor (dsu)
gene, the coat colour of dilute mice resembles that of
non-dilute mice28. dsu also suppresses the coat-colour
mutants ashen (Rab27a) and leaden (ln), as well as the
two ruby-eye colour mutants, ruby-eye (ru) and ruby-
eye 2 (ru2), but it does not affect the coat colour of several other mutants29. In dilute, ashen and leaden mutant
mice, melanocyte numbers are reduced and their dendritic processes are smaller, resulting in the retention of
pigment granules in the body of the melanocytes and
the characteristic dilute coat colour30. dsu increases
melanocyte numbers and restores normal morphology
and normal coat colour in dilute, ashen and leaden
mice. The mechanism by which dsu restores normal eye
colour in the ruby eye-colour mutants is uncertain and
the identity of the dsu gene is unknown.
Pleiotropy. Mice that are homozygous for the tubby
(tub) mutation are obese, deaf and blind31. Although
tubby mice are obese on all genetic backgrounds, hearing loss depends on unlinked modifier genes31. Tubby
mice obtained from intercrosses between a C57BL/6J
congenic strain that was homozygous for tub and either
of two unrelated strains were tested for hearing loss by
measuring their auditory brainstem responses. A gene,
called modifier of tubby hearing 1 (moth1), was shown
to control 50% of the genetic association of hearing loss
with obesity in tubby mice31. Thus an unlinked gene
controls hearing loss as a pleiotropic consequence of the
tubby mutation.
An example of a novel phenotype that is not found
in single mutant mice is the anatomical localization and
histological nature of spina bifida in mice mutant for
both patch and undulated32. Patch (Ph) is a deletion that
encompasses the platelet-derived growth factor receptor-α (Pdgfra) gene33 and undulated (un) is a mutation
in the Pax1 transcription factor34. Patch (Ph/+) heterozygotes lack pigmentation in the lumbosacral region;
homozygotes develop subcutaneous blisters that flank
the midline of the developing neural tube and do not
survive embryonic development because of SPINA BIFIDA
35,36
.
OCCULTA along the entire vertebral column
Undulated (un/un) homozygotes develop vertebral
defects that result in KYPHOSIS and tail kinks37,38. In addition to the pigmentation anomaly and the undulated
tail, which are expected in patch and undulated mice
with either of these mutations, all Ph/+ un/un doublemutant mice, and only the double mutants in these
crosses, have spina bifida occulta that is restricted to the
lower thoracic and lumbar regions. Histological analysis
indicates that the subcutaneous blisters that flank the
midline in patch homozygotes are moved to the midline, where they seem to interfere with vertebral fusion
in the lumbar region of these double-mutant mice32.
A closely related example involves neural tube defects
in splotch–curly-tail double mutant mice. Homozygous
Pax3Sp mice die mid-gestation from neural tube defects
that include RACHISCHISIS in either the hindbrain or lumbosacral region39–41. Curly-tail (ct) is a recessive mutation
that causes spina bifida (and occasionally ANENCEPHALY),
which is strikingly similar in its developmental characteristics to many cases of spina bifida in humans42,43. In
Pax3Sp ct/ct double-mutant mice, embryos develop with
neural tube defects that are more severe but different in
their developmental origins than those found in either
single mutant alone44. The phenotype of these double
NATURE REVIEWS | GENETICS
VOLUME 2 | MARCH 2001 | 1 6 9
© 2001 Macmillan Magazines Ltd
REVIEWS
Box 1 | Albinism — an unmodified trait?
L-phenylalanine
Albinism (oculocutaneous albinism
type 1, OCA1), which results from the
complete deficiency of the tyrosinase
protein, is one of the few examples of a
L-tyrosine
phenotype, the expression of which is
constant regardless of genetic
Tyrosinase
background. The tyrosine metabolic
pathway is involved in the synthesis of
L-dopa
Dopamine
eumelanin and phaeomelanin 85,86, and
a deficiency of tyrosinase in this
Tyrosinase
pathway results in the absence of these
pigments, as well as reduced vision,
Dopaquinone
nystagmus and photophobia. Mutant
Tyrosinase
+ Cysteine
alleles that retain some activity result
in hypopigmentation. The reason for
Quinones
5-S-, and
its lack of modification is thought to
2-S-cysteinyldopa
result from the structure of the
melanin synthesis pathway, the
position of tyrosinase in this pathway
Eumelanin
Phaeomelanin
and the nature of the molecular lesion
(black pigment)
(red pigment)
(see figure). Tyrosinase catalyses three
steps in this linear pathway that is
thought to consist of only four steps. In the absence of tyrosinase, there are no
metabolites that can act as targets for modification. The constancy of albinism
resulting from tyrosinase deficiency is unique among phenotypic traits, suggesting
that attributes of its function are unusual in mammalian biology and make it immune
to genetic modifiers. It is striking that, although deficiency of tyrosinase results in a
constant phenotype, mutations that affect the preceeding biochemical step, which
converts phenylalanine to tyrosine, result in substantial phenotypic variability4,5.
Phenylalanine
hydroxylase
mutant mice raises the possibility that an additive phenotypic effect does not necessarily reveal genes that interact
directly44. However, interactions, whether direct or indirect, cannot be excluded given our modest understanding
of many molecular, cellular and developmental processes.
Modifier genes in humans
Modifier effects are remarkably common in humans
and model organisms. In fact, it is probably exceptional
for phenotypes to present identically in all cases (for an
apparent exception, see BOX 1). In this part of the review,
I evaluate the evidence for modifier genes in humans
and restrict the discussion to those cases for which there
is evidence that independent genes control penetrance,
dominance, expressivity or pleiotropy of Mendelian
traits (TABLE 1). In the absence of this direct evidence, the
cause of phenotypic variability is uncertain.
RENAL AMYLOIDOSIS
A metabolic disorder associated
with the deposition of amyloid
(a protein–polysaccharide
complex) in the kidney.
SEROSAL INFLAMMATION
Inflammation of the
membranes that line the chest
and peritoneal cavities and that
enclose the lungs, the heart, the
main blood vessels and the gut.
170
Penetrance. Although reduced penetrance is a common
feature of many Mendelian traits, there are few available
examples of variability being caused by independent
genes. However, one example has recently been published
— a dominant modifier of the non-syndromic deafness
gene near DFNB26 (REF. 45). Most individuals who are
homozygous for the DFNB26 gene on chromosome 4q31
have non-syndromic hearing loss. However, several individuals in one of the families in which DFNB26 was
mapped are homozygous for the DFNB26 haplotype but
nevertheless have normal hearing. A dominant modifier
on chromosome 7 protects these individuals from hearing loss and accounts for reduced penetrance. Evidence
for a nuclear modifier of a mitochondrial disease has also
been recently reported46. A chromosome 8 gene, near
D8S277, has been reported to modify the penetrance of
the A1555G mutation in the mitochondrial 12S ribosomal gene, a mutation that causes maternally inherited
deafness46. Additional linkage studies are required to verify this provocative observation.
Dominance modification. Peripherin-1 (PRP1) is a gene
that exemplifies dominance modification. Some individuals that are heterozygous for PRP1 develop retinitis
pigmentosa, even though only homozygous individuals
are expected to be affected47. In these cases, alleles at the
unlinked rod outer segment protein 1 gene (ROM1) act
together with PRP1 variants to cause retinitis pigmentosa in double heterozygotes. So, an allele that normally
has a recessive effect on retinitis pigmentosa acts as a
dominant when a particular allele of the unlinked
ROM1 gene is present.
Expressivity. A possible example of a modifier gene that
reduces expressivity involves a gene that lowers cholesterol levels in individuals who are predisposed to familial
hypercholesterolaemia. Familial hypercholesterolaemia is
an autosomal dominant trait that affects one person in
500 and causes significantly elevated cholesterol levels48.
Familial hypercholesterolaemia heterozygotes are predisposed to premature cardiovascular disease and familial
hypercholesterolaemia homozygotes often die of cardiovascular disease during the first two decades of life. Some
individuals in familial hypercholesterolaemia families
have low density lipoprotein (LDL) levels that are ~25%
lower than expected, raising the possibility of a modifier
gene with cholesterol-lowering effects49. A recent study
mapped this modifier to chromosome 13q in several
populations, verifying the existence of a modifier gene
and demonstrating its general effects in these
populations50. However, this study also provided evidence
that the gene on 13q reduces the LDL level in individuals
with normal LDL levels, as well as those with familial
hypercholesterolaemia, raising the possibility that the 13q
gene has an additive rather than a modifier effect. In general, modifier genes that suppress disease could represent
an important way to exploit endogenous mechanisms to
suppress or perhaps even to prevent disease.
Pleiotropy. RENAL AMYLOIDOSIS is a variable complication
that is associated with familial Mediterranean fever51
(FMF). FMF is an autosomal recessive trait that is common in individuals in or derived from Mediterranean
populations and is characterized by the recurrence of
fever and SEROSAL INFLAMMATION. It results from a mutation
in the pyrin/marenostrin gene52. Amyloidosis is an
important complicating factor that can lead to renal
failure53. However, the variable association between
amyoidosis and FMF raises the possibility of genetic
modifiers. A recent study showed that a polymorphism
in serum amyloid A accounts in part for the variable
association of amyloidosis with FMF51.
| MARCH 2001 | VOLUME 2
www.nature.com/reviews/genetics
© 2001 Macmillan Magazines Ltd
REVIEWS
PANCREATIC INSUFFICIENCY
The inadequate functioning of
the pancreas that results from
the blockage of the pancreatic
duct, which prevents the
secretion of pancreatic fluids.
MECONIUM ILEUS
Obstruction of the intestine
by mucus.
Case studies of genetic modifiers
Modifier genes discovered in experimental species, such
as laboratory mice and rats, are often relevant to the
study of human diseases. Genetic linkages discovered in
one species can guide the search for linkage in humans,
as exemplified by the first example below. The second
example shows how the identity of modifier genes in
experimental species can reveal developmental or physiological pathways that modulate phenotypes in humans.
BILIARY CIRRHOSIS
Cirrhosis of the liver that results
from the obstruction and
inflammation of the bile ducts
and from the chronic retention
of bile.
Compensation for CFTR deficiency. Cystic fibrosis is
one of the most common human autosomal recessive
diseases54. Mutations in the cystic fibrosis transmembrane conductance regulator gene (CFTR), which
encodes a membrane-bound chloride ion channel,
cause impaired fluid and salt secretion in various tissues. These mutations can result in mucus accumulation in the airways, PANCREATIC INSUFFICIENCY, MECONIUM
ILEUS, BILIARY CIRRHOSIS, impaired salt resorption in sweat
glands, chronic bronchopulmonary infections and
infertility. The presentation of these clinical features of
cystic fibrosis can vary as can age-of-onset, disease
severity and other disease features. This variability
results partly from the numerous mutations that have
been identified in the CFTR gene54–56 and partly from
unlinked genetic modifiers57. In particular, few patients
with cystic fibrosis suffer from meconium ileus. The
lack of association between meconium ileus and specific
CFTR mutations raises the possibility that independent
genetic factors control the occurrence of meconium
ileus in patients with cystic fibrosis58–60 — an example of
pleiotropy modification.
To create a mouse model of cystic fibrosis, gene-targeting technology was used to make mice that are deficient in the mouse Cftr protein61–63. Homozygous
mutant mice die soon after birth because of mucus accumulation in the intestine61,62,64. Although failure to thrive
occurs on most genetic backgrounds, Cftr-deficient mice
live for many months on some backgrounds, implying
that modifier genes modulate disease severity65. Linkage
studies have revealed a single locus with semidominant
effects that account for most, but not all, of the variation
in viability. Cftr-deficient mice that do not have the
modifier gene die soon after birth, mice heterozygous for
the modifier gene usually survive until weaning, and
mice homozygous for the modifier survive for several
months longer than mice of the same Cftr genotype that
lack the modifier. This modifier reduces the amount of
mucus in the intestine and restores relatively normal
membrane electrophysiology, perhaps because of a calcium-activated chloride conductance channel that reduces
mucus accumulation in the intestine65. This cystic fibrosis modifier gene is located near the centromere of
mouse chromosome 7; the corresponding location in the
human genome is 19q13.2–q13.4. Associations were tested between meconium ileus, a pulmonary phenotype
and markers on this chromosome segment in patients
with cystic fibrosis. Strong evidence for linkage was
found for meconium ileus but not for the pulmonary
phenotype, suggesting that the occurrence of meconium
ileus with cystic fibrosis depends on mutations in this
chromosome 19 gene66. This is a striking example of a
modifier gene, the discovery of which in the mouse led
to the successful predictions of the location of a corresponding gene in humans.
Colon cancer susceptibility in Mom1 mice. Susceptibility
to ADENOMATOUS POLYPS characterizes the cancer predisposition syndrome called familial adenomatous polyposis
(FAP). It is an autosomal dominant trait with variable
penetrance and expressivity, which can be explained
partly by the hundreds of germline and somatic mutations in the adenomatous polyposis coli (APC) gene that
underlie the disease and partly by independent genetic
factors67. Mutations in the APC gene contribute to the
transformation of normal epithelial cells to benign adenomas, a critical step in the progression to colon cancer.
A survey of chemically mutagenized mice revealed an
induced mutant, originally called Min for ‘multiple
intestinal neoplasia’, with an increased incidence of
polyps68. Mapping studies identified the mouse homologue of APC as a candidate gene and sequencing studies
revealed a nonsense mutation in the mouse Apc gene69,
thereby establishing the ApcMin mouse as a model for
studies of FAP and the aetiology of spontaneous polyps.
The ApcMin mouse model has made important contributions to the development of preventative treatments for polyps because of a modifier gene that modulates polyp number and that was discovered as a result
of linkage studies to map the ApcMin mutant. During the
course of these mapping studies, William Dove and colleagues discovered that the number of polyps is
markedly reduced when ApcMin mice on a C57BL/6J
background are crossed to mice of the AKR strain70.
ApcMin mice on a C57BL/6J background have numerous
polyps, AKR/J mice with the ApcMin mutation have few
polyps, and F1 hybrids between the two strains (with the
ApcMin mutation) have an intermediate number of
polyps, indicating that a modifier gene with semidominant effects modulates polyp number. This modifier
gene, originally called Mom1 for ‘modifier of Min’, was
subsequently mapped to the distal portion of chromosome 4. It explains 50% of the genetic variance in polyp
number, indicating that other modifier genes with
weaker effects on polyp number probably exist.
The evaluation of candidate genes for Mom1
revealed a polymorphism in the secretory phospholipase A2 (Pla2g2a) gene. Pla2g2a belongs to a large family of phospholipases that hydrolyse phosphoglycerides
to yield fatty acids and lysophospholipids. This gene
family contributes not only to the metabolism of phospholipids but also to inflammatory responses. Strains
with the Mom1 variant that reduces polyp number
have a normal Pla2g2a gene, whereas strains with the
variant that fails to suppress polyp number have a deletion in the Pla2g2a gene and lack enzymatic activity71.
Complementation of high polyp number was evaluated in transgenic mice to test whether the normal
Pla2g2a gene accounts for the Mom1 phenotype72. As
expected, a significant reduction in polyp number was
observed. However, the reduction was not restored to
wild-type levels, indicating either that transgene
NATURE REVIEWS | GENETICS
VOLUME 2 | MARCH 2001 | 1 7 1
© 2001 Macmillan Magazines Ltd
REVIEWS
Box 2 | Finding modifier genes
Two general strategies can a
Strain A
Strain B
be used to discover
modifier genes in
X
laboratory mice. Typically,
an inbred strain that carries
+/+
–/–
a Mendelian mutation is
crossed to other inbred
strains and progeny are
F1 hybrid
F1 hybrid
examined for variation in
the mutant phenotype (see
X
figure). Panel a shows
segregation of a recessive
+/–
+/–
trait without modifiers,
panel b the segregation of
Phenotype
the same recessive trait, but frequencies:
75% Wild type
25% Mutant
in crosses with a strain that
has a single recessive
modifier gene (m). Another
X
approach is to transfer a
mutant gene from one
Genotype
25%
50%
25%
frequencies:
+/+
+/–
–/–
inbred strain background
to another inbred
b
F1 hybrid
F1 hybrid
background, through
repeated backcrossing and
selection to make a
X
‘congenic strain’. In these
cases, the phenotype
+/–
+/–
associated with the mutant
gene is evaluated on a
Phenotype
18.75%
frequencies:
75% Wild type
6.25%
different but defined
Unmodified mutant
Modified mutant
genetic background. Both
kinds of study have
provided classic examples
of modifier genes.
Genotype
25%
50%
6.25%
18.75%
Establishing that an
frequencies:
+/+
+/–
–/–
–/–
inbred strain has a modifier
(m/m)
gene is challenging, partly
because of lack of previous evidence that a particular strain has a modifier until a single gene defect is introduced and
partly because of the large sample sizes that are required. Generally, there is no way to know whether a particular strain
has a modifier without making the linkage crosses or congenic strains. As a result, several crosses involving many
progeny and many phenotype tests are needed, depending on the penetrance of the trait and the number of modifier
genes that are involved60,65. Alternatively, six to ten generations of backcrosses are needed to assess the mutant
phenotype on different genetic backgrounds. Information about which strains have, or do not have, the modifier gene, is
invaluable when evaluating candidate genes — strains with the modifier should have alleles that are distinct from those
in strains that do not have the modifier.
ADENOMATOUS POLYPS
Benign growths that arise from
the lining of the colon or
rectum, which can protrude
into the intestinal lumen.
172
expression might have failed to recapitulate fully wildtype functions or that other closely linked genes affect
polyp number and that the Mom1 phenotype results
from the cumulative action of several closely linked
genes. Although genes in the corresponding portion of
the human genome seem to have a modifier that
accounts for variation in FAP presentation73, polymorphisms in the homologue of the Pla2g2a gene do not
appear to account for significant variation in susceptibility to colon cancer74.
The finding that Pla2g2a modulates polyp number in
mice has nevertheless been informative given the pathway in which it acts. Pla2g2a is part of the prostaglandin
synthesis pathway. Aspirin, sulindac and other non-
steroidal anti-inflammatory agents markedly reduce susceptibility to polyps, perhaps by interfering with
cyclooxygenase-2 (Cox2) activity in the prostaglandin
pathway75–77. The involvement of Cox2 and the
prostaglandin pathway is supported by the observation
that Apcmin mice deficient for either Cox2 or cytosolic
phospholipase A2 (cPLA2) have reduced polyp numbers78,79. Although Mom1 did not identify a gene that
modulates susceptibility to colon cancer to a significant
extent in humans, it did successfully identify a pathway
that is pharmacologically relevant to the management of
colon cancer. So genetic variants in mice do not also
need to identify genetic variants in humans for them to
be useful in understanding and treating human diseases.
| MARCH 2001 | VOLUME 2
www.nature.com/reviews/genetics
© 2001 Macmillan Magazines Ltd
REVIEWS
Conclusion
A recent paper questions the impact of genetics and
genomics on health care because penetrance confounds
genetic analysis of simple and especially of complex
genetic traits80. Although this viewpoint is unnecessarily
pessimistic81–84, the burden of proof should rightly be on
the genetics community to demonstrate the contribution that genetics, and genetic modifiers in particular,
can make to the diagnosis, treatment and prevention of
common human diseases. The evidence for modifier
genes in humans and mice, and examples of mouse
modifier genes that have guided the search for modifier
genes in humans and led to therapeutically relevant biochemical pathways, suggests that factors that complicate
inheritance of Mendelian traits can be effectively studied to great advantage.
The availability of the sequence of the mouse
genome will resolve a key challenge in establishing the
identity of modifier genes (current strategies for identifying modifier genes are summarized in BOX 2).
These modifiers, which are interesting as phenotypic
phenomena, illustrate the importance of dissecting
the complex networks of gene interactions that
underlie variation in organismal biology. In particular, the identities of the modifier and modified genes
provide insights into the molecular, biochemical and
cellular mechanisms for modification. With the
reported completion of the sequencing of a draft version of the mouse genome (see link to Celera), large
numbers of genetic markers will become available
with which to determine the precise chromosomal
location of genetic modifiers. The sequenced mouse
genome, together with the sequence of the human
genome, should allow the location of the corresponding gene in the human genome to be accurately predicted. The sequence of the mouse and human
genomes will revolutionize the analysis of modifier
genes and variable phenotypes.
1.
Bridges, C. B. Specific modifiers of eosin eye color in
Drosophila melanogaster. J. Exp. Zool. 10, 337–384
(1919).
2. Gruneberg, H. The Pathology of Development (John Wiley,
New York, 1950).
3. Romeo, G. & McKusick, V. A. Phenotypic diversity, allelic
series and modifier genes. Nature Genet. 7, 451–453
(1994).
4. Scriver, C. R. & Waters, P. J. Monogenic traits are not
simple: lessons from phenylketonuria. Trends Genet. 15,
267–272 (1999).
5. Dipple, K. M. & McCabe, E. R. B. Phenotypes of patients
with ‘simple’ Mendelian disorders are complex traits:
thresholds, modifiers, and system dynamics. Am. J. Hum.
Genet. 66, 1729–1735 (2000).
6. Feingold, J. Les genes modificateurs dans les maladies
hereditaires. Medicine/sciences 16, 1–5 (2000).
7. Davis, A. & Justice, M. J. Mouse alleles: if you’ve seen
one, you haven’t seen them all. Trends Genet. 14,
438–441 (1998).
8. McGue, M. & Bouchard, T. J. Genetic and environmental
influences on human behavioral differences. Annu. Rev.
Neurosci. 21, 1–24 (1998).
9. Cookson, W. The alliance of genes and environment in
asthma and allergy. Nature 402, B5–B11 (1999).
10. Perusse, L. & Bouchard, C. Genotype–environment
interaction in human obesity. Nutr. Rev. 57, S31–S37
(1999).
11. Shields, P. G. & Harris, C. C. Cancer risk and low-
Modifier genes provide insights into the mechanisms
by which organisms modulate biological processes to
accommodate the adverse effects of genetic mutations.
Carrier individuals that are free of disease or that show a
substantially ‘normal’ phenotype are examples of developmental and physiological accommodation of disease
risk despite inherited susceptibility to disease50,51,66. These
modifier genes often have at least two alleles, one of
which exacerbates disease, and one that suppresses disease. These alleles can be thought of as being ‘disease
promoting’ and ‘disease suppressing’, respectively. These
disease-suppressing modifier genes move the phenotypic
threshold for expression of traits so that fewer carrier
individuals are affected. New disease therapeutics could
be based on mimicking and perhaps enhancing the
effects of naturally occurring genetic modifiers.
Understanding the basis for variable disease presentation
in general, and for the suppression of disease in particular, could improve the prediction, treatment and perhaps
even the prevention of human diseases.
Links
DATABASE LINKS disorganization | undulated | short-ear |
Pax1 | Bmp5 | mdfw | T mutation | brachyury | splotch |
Pax3 | Fidget | Fign | dilute suppressor mutation | Myo5a |
dsu | ashen | Rab27a | leaden | ln | ruby-eye | ru | ruby-eye 2
| ru2 | tubby | Patch | Pdgfrα | undulated | Curly-tail |
DFNB26 | mitochondrial 12S ribosomal gene | PRP1 |
retinitis pigmentosa | ROM1 | familial
hypercholesterolaemia | familial Meditteranean fever |
pyrin/marenostrin | serum amyloid A | cystic fibrosis |
CFTR | Cftr | familial adenomatous polyposis | APC | Min |
Apc | Pla2g2a | Cox2 activity in the prostaglandin pathway
| Cox2 | cytosolic phospholipase A2
FURTHER INFORMATION Mouse genome sequencing at
Celera
ENCYCLOPEDIA OF LIFE SCIENCES Genotype–phenotype
relationships
penetrance susceptibility genes in gene–environment
interactions. J. Clin. Oncol. 18, 2309–2315 (2000).
12. Fields, S. & Sternglanz, R. The two-hybrid system: an
assay for protein–protein interactions. Trends Genet. 10,
286–292 (1994).
13. Husi, H. et al. Proteomic analysis of NMDA receptoradhesion protein signaling complexes. Nature Neurosci. 3,
661–669 (2000).
14. King, R. C. & Stansfield, W. D. A Dictionary of Genetics
(Oxford Univ. Press, Oxford, 1990).
15. Phillips, P. C. The language of gene interaction. Genetics
149, 1167–1171 (1998).
16. Hummel, K. P. The inheritance and expression of
Disorganization, an unusual mutation in the mouse. J. Exp.
Zool. 137, 389–423 (1958).
17. Hummel, K. P. Developmental anomalies in mice resulting
from action of the gene, Disorganization, a semi-dominant
lethal. Pediatrics 23, 212–221 (1959).
18. Crosby, J. L., Varnum, D. S. & Nadeau, J. H. Two-hit
model for sporadic congenital anomalies in mice with the
disorganization mutation. Am. J. Hum. Genet. 52,
866–874 (1993).
This paper shows how birth defects with sporadic
features can be accounted for by the random
occurrence of adverse events in somatic tissues, as
occurs in many cancers.
19. Gruneberg, H. Genetic studies on the skeleton of the
mouse. II. Minor variations of the vertebral column. J.
Genet. 50, 112–141 (1950c).
NATURE REVIEWS | GENETICS
20. Gruneberg, H. Genetical studies on the skeleton of the
mouse. XV. Relations between major and minor variants. J.
Genet. 53, 515–535 (1955).
21. Noben-Trauth, K., Zheng, Q. Y., Johnson, K. R. & Nishina,
P. M. mdfw: a deafness susceptibility locus that interacts
with deaf waddler (dfw). Genomics 44, 266–272 (1997).
22. Dobrovolskaia-Zavadskaia, N. Sur une novelle mutation de
la queue, ‘queue filiforme’, chez la souris. C. R. Soc. Biol.
Paris 97, 1140–1142 (1927).
23. Dunn, L. C. Changes in the degree of dominance of
factors affecting tail-length in the house mouse. Am. Nat.
76, 552–569 (1942).
24. Cox, G. A. et al. The mouse fidgetin gene defines a new
role for AAA family proteins in mammalian development.
Nature Genet. 26, 198–202 (2000).
25. Gruneberg, H. Two new mutant genes in the house
mouse. J. Genet. 45, 22–28 (1943).
26. Konyukhov, B. V. & Mironova, O. V. Interaction of the
mutant genes splotch and fidget in mice. Sov. Genet. 15,
553–562 (1979).
27. Wilson, D. B. Proliferation of the neural tube of the Splotch
(Sp) mutant mouse. J. Comp. Neurol. 154, 249–256
(1974).
28. Sweet, H. O. Dilute suppressor, a new suppressor gene in
the house mouse. J. Hered. 74, 305–306 (1983).
29. Moore, K. J., Swing, D. A., Copeland, N. G. & Jenkins, N.
A. Interaction of the murine dilute suppressor gene (dsu)
with fourteen color mutations. Genetics 125, 421–430
(1990).
VOLUME 2 | MARCH 2001 | 1 7 3
© 2001 Macmillan Magazines Ltd
REVIEWS
30. Moore, K. J. et al. The murine dilute suppressor gene dsu
suppresses the coat-color phenotype of three pigment
mutations that alter melanocyte morphology, d, ash, and
ln. Genetics 119, 933–941 (1988).
References 29 and 30 discuss the characterization
of one of the few known suppressor mutations in
mice.
31. Ikeda, A. et al. Genetic modification of hearing in tubby
mice: evidence for the existence of a major gene (moth1)
which protects tubby mice from hearing loss. Hum. Mol.
Genet. 8, 1761–1767 (1999).
32. Helwig, U. et al. Interaction between undulated and Patch
leads to an extreme form of spina bifida in double-mutant
mice. Nature Genet. 11, 60–63 (1995).
33. Smith, E. A. et al. Mouse platelet-derived growth factor
receptor-α gene is deleted in W19H and patch mutations
on chromosome 5. Proc. Natl Acad. Sci. USA 88,
4811–4815 (1991).
34. Balling, R. et al. Undulated, a mutation affecting the
development of the mouse skeleton, has a point mutation
in the paired box of Pax1. Cell 55, 531–555 (1988).
35. Gruneberg, H. & Truslove, G. Two closely linked genes in
the mouse. Genet. Res. 1, 69–90 (1960).
36. Payne, J., Shibasaki, F. & Mercola, M. Spina bifida occulta
in homozygous Patch mouse embryos. Dev. Dynam. 209,
105–116 (1997).
37. Wright, M. E. Undulated, a new genetic factor in Mus
musculus affecting the spine and tail. Heredity 1, 137–141
(1947).
38. Gruneberg, H. Genetical studies on the skeleton of the
mouse. II. Undulated and its ‘modifiers’. J. Genet. 50,
142–173 (1950).
39. Russell, W. L. Splotch, a new mutation in the house
mouse, Mus musculus. Genetics 32, 102 (1947). [LAST
PAGE?]
40. Epstein, D. J., Vekemans, M., & Gros, P. Splotch (Sp2H), a
mutation affecting development of the mouse neural tube,
shows a deletion within the paired homeodomain of Pax-3.
Cell 67, 767–774 (1993).
41. Goulding, M. et al. Analysis of the Pax-3 gene in the
mouse mutant Splotch. Genomics 17, 355–363 (1993).
42. Neumann, P. E. et al. Multifactorial inheritance of neural
tube defects: localization of the major gene and
recognition of modifiers in ct mutant mice. Nature Genet.
6, 357–362 (1994).
43. Copp, A. J. Genetic models of mammalian neural tube
defects. Ciba Found. Symp. 181, 118–134 (1994).
44. Estibeiro, J. P. et al. Interactions between splotch (Sp) and
curly-tail (ct) mouse mutants in the embryonic
development of neural tube defects. Development 119,
113–121 (1993).
45. Riazuddin, S. et al. Dominant modifier DFNM1 suppresses
recessive deafness in DFNB26. Nature Genet. 26,
431–434 (2000).
46. Bykhovskaya, Y. et al. Candidate locus for a nuclear
modifier gene for maternally inherited deafness. Am. J.
Hum. Genet. 66, 1905–1910 (2000).
47. Kajiwara, K., Berson, E. L. & Dryja, T. P. Digenic retinitis
pigmentosa due to mutations at the unlinked
peipherin/RDS and ROM1 loci. Science 264, 1604–1608
(1994).
This is a classic example of digenic inheritance —
when alleles of two genes are needed for the trait to
appear.
48. Brown, M. L. & Goldstein, J. L. A receptor-mediated
pathway for cholesterol homeostasis. Science 232, 34–47
174
(1986).
49. Hobbs, H. et al. Evidence for a dominant gene that
suppresses hypercholesterolemia in a family with defective
low density lipoprotein receptors. J. Clin. Invest. 84,
656–664 (1989).
50. Knoblauch, H. et al. A cholesterol lowering drug maps to
chromosome 13q. Am. J. Hum. Genet. 66, 157–66
(2000).
Although it is uncertain whether the gene(s) on
chromosome 13q has a modifier or an additive
effect, this paper shows the importance of studying
the various ways in which traits and diseases can
present, and illustrates the potential therapeutic
relevance of genes that suppress clinically important
phenotypes.
51. Cazeneuve, C. et al. Identification of MEFV-independent
modifying genetic factors for familial mediterranean fever.
Am. J. Hum. Genet. 67, 1136–1143 (2000).
One of the few examples of linkage to a modifier
gene in humans.
52. Booth, D. R. et al. Pyrin/marenostrin mutations in familial
Mediterranean fever. Quart. J. Med. 91, 603–606 (1998).
53. Sohar, M. et al. Phenotype–genotype correlation in familial
Mediterran fever: evidence for an association between
Met694Val and amyloidosis. Eur. J. Hum. Genet. 7,
287–292 (1999).
54. Welsh, M. J., Ramsey, B. W., Accurso, F. & Cutting, G. R.
in The Metabolic and Molecular Bases of Inherited Disease
(eds Scriver, C. R., Beaudet, A. L., Sly, W. S. & Valle, D.)
5121–5188 (McGraw-Hill, New York, 2001).
55. Zielenski, J. & Tsui, L.-C. Cystic fibrosis: genotype and
phenotype variations. Annu. Rev. Genet. 29, 777–807
(1995).
56. Zielenski, J. Genotype and phenotype in cystic fibrosis.
Respiration 67, 117–133 (2000).
57. Knowles, M. R. et al. Mild cystic fibrosis in a
consanguinous family. Ann. Intern. Med. 110, 1410–1417
(1989).
58. Kerem, E. et al. Clinical and genetic comparisons of
patients with cystic fibrosis, with or without meconium
ileus. J. Pediat. 114, 767–773 (1989).
59. Kristidis, P. & Tsui, L.-C. Genetic determination of exocrine
pancreatic function in cystic fibrosis. Am. J. Hum. Genet.
50, 1178–1184 (1992).
60. Cystic Fibrosis Genotype–Phenotype Consortium.
Correlation between genotype and phenotype in patients
with cystic fibrosis. N. Engl. J. Med. 329, 1308–1313
(1993).
61. Snouwaert, J. N. et al. An animal model of cystic fibrosis
made by gene targeting. Science 257, 1083–1088 (1992).
62. Dorin, J. R. et al. Cystic fibrosis in the mouse by targeted
insertional mutagenesis. Nature 359, 211–215 (1992).
63. Zeiher, B. G. et al. A mouse model for the delta F508 allele
of cystic fibrosis. J. Clin. Invest. 96, 2051–2064 (1995).
64. Colledge, W. H. et al. Cystic fibrosis mouse with intestinal
obstruction. Lancet 340, 680 (1992).
65. Rozmahel, R. et al. Modulation of disease severity in cystic
fibrosis transmembrane conductance regulator deficient
mice by a secondary genetic factor. Nature Genet. 12,
280–287 (1996).
This study shows the importance of genetic
modifiers in mouse models of human disease. It was
the first example of linkage to a modifier that was
discovered in mice and then replicated in humans.
66. Zielenski, J. et al. Detection of a cystic fibrosis modifier
locus for meconium ileus on human chromosome 19q13.
| MARCH 2001 | VOLUME 2
Nature Genet. 22, 128–129 (1999).
67. Beroud, C. & Soussi, T. APC gene: database of germline
and somatic mutations in human tumors and cell lines.
Nucleic Acids Res. 24, 121–124 (1996).
68. Moser, A. R., Pitot, H. & Dove, W. F. A dominant mutation
that predisposes to multiple intestinal neoplasia in the
mouse. Science 247, 322–324 (1990).
69. Su, L.-K. et al. Multiple intestinal neoplsia caused by a
mutation in the murine homolog of the APC gene. Science
256, 668–670 (1992).
70. Dietrich, W. F. et al. Genetic identification of Mom-1, a
major modifier locus affecting Min-induced intestinal
neoplasia in the mouse. Cell 75, 631–639 (1993).
A classic paper on the genetics of modifier genes,
which was based on genetic discoveries in mice and
successfully identified a pharmacologically relevant
pathway in humans.
71. MacPhee, M. et al. The secretory phospholipase A2 gene
is a candidate for the Mom1 locus, a major modifier of
ApcMin-induced intestinal neoplasia. Cell 81, 957–966
(1995).
72. Cormier, R. T. et al. Secretory phospholipase Pla2g2a
confers resistance to intestinal tumorigenesis. Nature
Genet. 17, 88–91 (1997).
73. Dobbie, Z. et al. Identification of a modifier gene locus on
chromosome 1p35–36 in familial adenomatous polyposis.
Hum. Genet. 99, 653–657 (1997).
74. Nimmrich, I. et al. Loss of the PLA2G2A gene in a sporadic
colorectal tumor of a patient with a PLA2G2A germline
mutation and absence of PLA2G2A germline alterations in
patients with FAP. Hum. Genet. 100, 345–349 (1997).
75. Torrance, C. J. et al. Combinatorial chemoprevention of
intestinal neoplasia. Nature Med. 6, 1024–1028 (2000).
76. Taketo, M. Cyclooxygenase-2 inhibitors in tumorigenesis
(Part 1). J. Natl Canc. Inst. 90, 1529–1536 (1998).
77. Taketo, M. Cyclooxygenase-2 inhibitors in tumorigenesis
(Part II). J. Natl Canc. Inst. 90, 1609–1620 (1998).
78. Oshima, M. et al. Suppression of intestinal polyposis in
Apc∆716 knockout mice by inhibition of cyclooxygenase 2
(Cox-2). Cell 87, 803–809 (1996).
79. Takaku, K. et al. Suppression of intestinal polyposis in
Apc∆716 knockout mice by an additional mutation in the
cytosolic phospholipase A2 gene. J. Biol. Chem. 275,
34013–34016 (2000).
80. Holtzman, N. A. & Marteau, T. M. Will genetics
revolutionize medicine? N. Engl. J. Med. 343, 141–144
(2000).
81. Sotos, J. G. & Rienhoff, H. Y. Letter. N. Engl. J. Med. 343,
1496 (2000).
82. Block, G. D. & Aulisio, M. P. Letter. N. Engl. J. Med. 343,
1496 (2000).
83. Khoury, M. J. Letter. N. Engl. J. Med. 343, 1497 (2000).
84. Ference, B. A., Chauhan, M. S. & Izumo, S. Letter. N. Engl.
J. Med. 343, 1497–1498 (2000).
85. Silvers, W. K. The Coat Colors of Mice (Springer, New
York, 1979).
86. Michal, G. Biochemical Pathways (John Wiley, New York,
1998).
Acknowledgements
I thank A. Matin for insightful discussions about modifier genes, N.
Robin for sharing examples of modifier genes, and M. Drumm and
M. Taketo for comments on portions of this manuscript. This work
was supported by grants from the NIH, by a grant from the Keck
Foundation to the Department of Genetics, and by a grant from the
Howard Hughes Medical Institute to the Case Western Reserve
University School of Medicine.
www.nature.com/reviews/genetics
© 2001 Macmillan Magazines Ltd